The objectives of the study are: To compare the response rate of patients receiving rIFN-ylb in conujunction with calcitriol to the response rate of patients receiving calcitriol alone. To determine the safety of rIFN-ylb administered to patients with osteopetrosis. To relate alterations in biologic response markers to disease outcome. The data to date has smaller numbers than the previous trials. There has been a trend towards an increase in bone turnover and cranial foraminal size. However, due to death and withdrawal for transplantations, the number of patients who have completed the trial is small. The most important accomplishment has been the recruitment of additional patients from Saudi Arabia (3) with the expectation to enroll 6-12 more. The enrollment rate of the US along with the Saudi Arabian enrollment should exceed the enrollment rate by the end of this year, and to move forward towards final approval.